The briard dog is affected with a recessively inherited retinal disorder characterized by congenital night blindness with various degrees of visual impairment under photopic illumination. Vision in affected dogs ranges from normal day vision to profound day blindness (Narfstrom et al., "Hereditary Retinal Dystrophy In The Briard Dog: Clinical And Hereditary Characteristics," Veterinary & Comparative Ophthalmology, 4:85-92 (1994)). The disease was initially described in Swedish dogs as a stationary disorder analogous to human congenital stationary night blindness (CSNB; Narfstrom et al., "Hereditary Retinal Dystrophy In The Briard Dog: Clinical And Hereditary Characteristics," Veterinary & Comparative Ophthalmology, 4:85-92 (1994)). More recently, the disease has been described as having a progressive component, and has been termed hereditary retinal dystrophy (Wrigstad et al., "Slowly Progressive Changes Of The Retina And Retinal Pigments Epithelium In Briard Dogs With Hereditary Retinal Dystrophy. A Morphologic Study," Doc. Ophthalmol., 87:337-354 (1994); Wrigstad, "Hereditary Dystrophy Of The Retina And The Retinal Pigment Epithelium In A Strain Of Briard Dogs: A Clinical, Morphologic And Electroretinographic Study," Linkoping University Medical Dissertation #423 (1994)). Along with the visual impairment, affected dogs have an abnormal electroretinogram (ERG); in general, the recorded responses are normal in waveform, but show a marked diminution of response amplitudes, similar to a "Riggs type" ERG in man. The ERG recorded under DC conditions shows complete absence of the a-, b-, and c-waves, with the latter waveform being replaced by a very slow negative potential which develops when the stimulus intensity is greater than 3 log units above the normal b-wave threshold. The abnormalities in the a- and b-waves can be interpreted as representing a delay in rod phototransduction (Nilsson et al., "Changes In The DC Electroretinogram In Briard Dogs With Hereditary Congenital Night Blindness And Partial Day Blindness," Exp. Eye Res., 54:291-296 (1992)). A similar disease is also recognized in other countries, e.g. France, Canada, and the United States. In the United States, the disease is termed congenital stationary nightblindness, and csnb has been designated as the gene symbol for the disease locus. Apart from the above studies in Swedish briard dogs, no other systematic investigation of the disease has been reported, nor has there been definitive proof that csnb and retinal dystrophy represent the same disorder.
The present invention is directed to overcoming the above-noted deficiencies in the prior art.